The acre is a unit of area used in the imperial and U.S. customary systems. An acre is about 40% of a hectare – slightly smaller than an American football field. The acre is no longer commonly used, although a few notable exceptions include the United States, Australia, Burma and the United Kingdom (as of 2010, the acre is no longer officially used in the United Kingdom, though is still used in real estate descriptions). It is still used, to some extent, in Canada.